Preserving tree health in the face of soilborne diseases and pests requires both careful management and choosing appropriate rootstock genetics for a given orchard site.
In a West Coast Nut MyAgLife webinar on soilborne pests and diseases in nut crops, several experts in soilborne diseases and rootstock development shared their knowledge and experience on a number of common soilborne pathogens in walnuts and almonds, as well as breeding efforts to find solutions through resistant rootstock.
Diseases in Almonds
Mohammad Yaghmour, orchard systems advisor in Kern County, said there has been a resurgence in recent years in Phytophthora, one of the most aggressive soilborne pathogens, in Kern County.
“It is the most important soilborne disease we face every year,” Yaghmour said.
There are several species of this pathogen which cause root, crown, trunk and scaffold cankers. Infected trees lose vigor and eventually collapse. Sources of the infection can include infected plant material and surface irrigation water. The disease is best managed through prevention, by avoiding over-irrigation and standing water in low spots in the orchard and by planting on elevated berms.
In almond orchard sites, where Phytophthora has been found, Yaghmour said, using resistant rootstocks is advised. Preventative foliar sprays are another control option. Applying a fall or spring spray of phosphonate has shown to have a significant effect, suppressing development of cankers for up to five months after the treatment. The sprays will not eradicate the pathogens from the tree, Yaghmour emphasized.
It is important to meet nutrition needs of trees and to avoid stress, Yaghmour said, as stress and poor nutrition predispose trees to diseases.
Other soilborne diseases that can affect almonds include verticillium and armillaria root rot. Verticillium, also known as blackheart, is a vascular disease that mainly affects young trees. Infections can be obvious after a heat event in late spring or early summer. There will be flagging on one side of the tree and the shoot tips will resemble a shepherd’s hook. A cross section will show discoloration that extends to the origin of the infection in the root.
Knowing the site history is important as the pathogen may be in the soil. A preplant fumigation or solarization can reduce the level of inoculum in the soil, but it will not eliminate all of the sclerotia. Verticillium has many hosts, with cotton and tomatoes being highly susceptible hosts. These should be avoided as intercrops.
A less common but still devastating disease is Armillaria root rot. Almond orchards adjacent to riparian areas or oak trees are more likely to be affected. This disease is spread from tree to tree through root grafts, and introduced at a site via infested soil or plant material. Affected trees have thin canopies and pale foliage. They lack new growth, and the trees may wilt and die during hot weather. Inspection of the roots shows the presence of mycelial mats between the bark and wood tissue. Exclusion of infested soils on equipment and removing old roots from the orchard before replanting are the best strategies for control of this disease.
Yaghmour emphasized the importance of confirmation of a disease with laboratory tests as symptoms are not diagnostic and may be confused with other maladies.
Soilborne Issues in Walnuts
The second portion of the webinar focuses on soilborne diseases of walnut. Westphal summarized the effort of a research consortium including plant scientists, geneticists, engineers and economists. Supported by federal, state and California Walnut Board funding, this group set out to improve walnut rootstocks with improved capacities to resist soilborne diseases. Selections for elevated resistance to Phytophthora rots, crown gall, root lesion and root knot nematodes are underway.
USDA-ARS plant pathologist Greg Browne explained that Phytophthora is an oomycete that differs from true fungi. Water status is central to its biology and pathology. Soil water saturation favors zoospore production, dispersal and attraction to plant roots. Due to these characteristics, Phytophthora infections in an orchard can explode, Browne said.
Regionally, surface sources of irrigation water, including rivers and canals, can spread Phytophthora species. Ponds and reservoirs can support the pathogen. Browne emphasized that growers should be aware of the potential of infection from these water sources and manage accordingly.
In California, Phytophthora cinnamomi is the most aggressive species in walnuts. In an aerial photo, Browne noted the devastation in a walnut orchard due to Phytophthora at a site that was fumigated with methyl bromide prior to planting. Susceptible rootstock and soil characteristics contributed to the severity of the infection.
When this species infects trees in an orchard, Browne said it invades the roots first, then the crown and trunk. A bleeding trunk is not diagnostic for a Phytophthora infection because other diseases or disorders can cause similar symptoms.
The other major species is Phytophthora citricola. It is also an aggressive pathogen, but it is more of a “crown rotter.”
It is important to confirm suspicions of the disease based on symptoms by laboratory procedures. Browne noted that ELISA test results can be misleading and a laboratory test either using morphological or molecular procedures should be done. Paradox canker, for example, looks similar to Phytophthora, but is of unknown etiology.
Genetic resistance in rootstocks is an important factor for Phytophthora suppression. Browne noted the currently available walnut rootstocks and their resistance levels. The clonal walnut rootstock RX1, even in field trials, is showing high resistance to P. cinnamomi and moderate to high resistance to P. citricola. English seedling or clonal and northern black seedling both have very low resistance. Paradox seedling has variable resistance from low to moderate in both species. Vlach clonal has low resistance to both, and VX211 has low resistance to cinnamomi and moderate resistance to citricola.
Next-generation rootstocks and crown gall resistance was addressed by USDA–ARS researcher Dan Kluepfel. The objectives of this project are to generate a genetically diverse Juglans germplasm collection, identify hybrid Juglans germplasm resistant to crown gall (Agrobacterium tumefaciens), and then generate and clonally propagate hybrid disease-resistant genotypes for validation in field trials with a goal of delivering disease resistant rootstocks to growers.
This pathogen, Kluepfel explained as he showed how crown gall resistance is determined, requires a wound to enter a tree. Wound sites inoculated with Agrobacterium tumefaciens, the causative agent of crown gall, are rated for the appearance of galls after 2 to 4 months or, in some cases, after a dormancy period. A No. 1 rating means no symptoms (i.e. resistant), No. 2 is gall symptoms with stem girdling less than 25%, No. 3 is gall symptoms and stem girdling between 25 to 50% and No. 4 is gall symptoms with stem girdling more than 50%. In this way, the novel walnut genotypes are categorized based on their response to the crown gall pathogen. Selections with a rating of 1 or 2 are re-propagated and re-examined under both greenhouse and field conditions to confirm resistance.
Currently, there are five large-scale field trials evaluating “elite” putative resistant rootstock germplasm. The trials are in Glenn, Sutter, Lake, Solano and Tulare counties. At the Solano trial, the trees are being inoculated with the pathogens for crown gall and Phytophthora. At the other sites, the trees are planted in naturally infested soils.
Kluepfel said that 25 other putative disease-resistant hybrids have been clonally propagated and will be ready for field testing in 2020-2021.
Root Lesion Nematode
Tolerance and resistance determination for nematodes was covered by Nematologist Andreas Westphal of UC Riverside. There are other nematode species that can harm walnut tree roots, but root lesion nematodes are the most damaging. Westphal explained that the definition of resistance to this nematode depends on its ability to reproduce on the roots. If it cannot, the rootstock is resistant. Tolerance is different in that it is the plant’s response to the presence of the nematode. Plants can be tolerant to this nematode, but they can also be susceptible.
Screening rootstocks for nematode resistance and tolerance involves planting clonal hybrids and inoculating the plants, monitoring growth and sampling the roots for damage. Westphal said huge differences in plant responses have been seen in just the first year. But in his work, he confirmed that a second year of field cultivation is necessary to find meaningful numbers of root infections. After one year of growth, it can happen that 50-60% of the test plants have low nematode numbers in the roots – misleading to rate them “resistant”. But in the second year, only 2-3% of these same genotypes may only have low nematode numbers in the root nullifying the erroneous assumptions of the first year. Thus, Westphal’s testing always takes at least two years.
Work by this group of plant pathologists and researchers continues to provide California walnut growers with the genetics proven to ensure good walnut production.